1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for allocating subchannels to a plurality of mobile stations (MSs) in an orthogonal frequency division multiple access (OFDMA) mobile communication system.
2. Description of the Related Art
A third generation (3G) mobile communication system is evolving into a fourth generation (4G) mobile communication system. The 4G mobile communication system is currently under the standardization process for the purpose of providing an efficient interworking and an integrated service between a wired communication network and a wireless communication network beyond the simple wireless communication service that the previous-generation mobile communication systems provide.
A signal on a radio channel experiences multipath interference due to obstacles encountered between a transmitter and a receiver in the mobile communication system. The multipath radio channel is characterized by its maximum delay spread and signal transmission period. If the maximum delay spread is longer than the transmission period, there is no interference between successive signals and the channel is characterized as a frequency nonselective fading channel.
However, the use of a single carrier scheme for high-speed data transmission with a short symbol period leads to severe intersymbol interference (ISI). The resulting increased signal distortion increases the complexity of an equalizer at a receiver.
In this context, orthogonal frequency division multiplexing (OFDM) was proposed as a useful scheme for solving the equalization problem in the single carrier transmission scheme.
OFDM is a special case of multi-carrier modulation (MCM) in which a serial symbol sequence is converted to parallel symbol sequences and modulated to a plurality of mutually orthogonal subcarriers (or sub-carrier channels).
The first MCM systems appeared in the late 1950's for military high frequency (HF) radio communication, and OFDM with overlapping orthogonal sub-carriers was initially developed in the 1970's. In view of the orthogonal modulation between the multiple carriers, the OFDM has limitations in the actual implementation for the systems. In 1971, Weinstein, et. al. proposed an OFDM scheme that applies a DFT (Discrete Fourier Transform) to the parallel data transmission as an efficient modulation/demodulation process, which was a driving force behind the development of the OFDM. Also, the introduction of a guard interval and a cyclic prefix as the guard interval further mitigates many of the adverse effects of the multi-path propagation and the delay spread on the systems.
That is why OFDM has been widely exploited for digital data communications such as digital audio broadcasting (DAB), digital TV broadcasting, wireless local area network (WLAN), and wireless asynchronous transfer mode (WATM). Although the complexity of the hardware was an obstacle to the wide use of the OFDM, recent advances in digital signal processing technology including FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform) have enabled the OFDM to be more easily implemented.
OFDM, similar to FDM (Frequency Division Multiplexing), boasts of an optimum transmission efficiency in a high-speed data transmission because it transmits the data on sub-carriers, maintaining an orthogonality among them. The optimum transmission efficiency is further attributed to a good frequency use efficiency and a robustness against the multi-path fading in the OFDM.
Overlapping frequency spectrums lead to an efficient frequency use and a robustness against frequency selective fading and multi-path fading. The OFDM reduces the effects of the ISI by using guard intervals, and enables the design of a simple equalizer hardware structure. Furthermore, since the OFDM is robust against impulse noise, it is increasingly popular in communication systems.
With reference to FIG. 1, the structure of a typical OFDM mobile communication system will be described.
FIG. 1 is a block diagram of the typical OFDM communication system. The OFDM communication system is composed of a transmitter 100 and a receiver 150.
Referring to FIG. 1, the transmitter 100 comprises an encoder 104, a symbol mapper 106, a serial-to-parallel converter (SPC) 108, a pilot symbol inserter 110, an IFFT 112, a parallel-to-serial converter (PSC) 114, a guard interval inserter 116, a digital-to-analog converter (DAC) 118, and a radio frequency (RF) processor 120.
Upon the generation of the user data bits and the control data bits to be transmitted, the data and control bits are provided to the encoder 104. The user data bits and control data bits are collectively referred to as “user data” 102. The encoder 104 encodes the user data 102 according to a predetermined coding method. The coding method can be, but is not limited to, turbo coding or convolutional coding with a predetermined coding rate. The symbol mapper 106 modulates the coded bits received from the encoder 104 according to a predetermined modulation scheme. The modulation scheme can be, but is not limited to, BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 16 QAM (16ary Quadrature Amplitude Modulation), or 64 QAM.
The SPC 108 converts the serial modulated symbol sequence received from the symbol mapper 106 to parallel symbol sequences. The pilot symbol inserter 110 inserts pilot symbols into the parallel modulated symbols. The IFFT 112 N-point IFFT-processes the signal received from the pilot symbol inserter 110.
The PSC 114 serializes the IFFT signal. The guard interval inserter 116 inserts a guard interval into the serial signal from the PSC 114. The guard interval is inserted in order to cancel the effect of the interference between an OFDM symbol transmitted for the previous OFDM symbol period and an OFDM symbol to be transmitted for the current OFDM symbol period. It was initially proposed that null data is inserted for a predetermined period as the guard interval. However, the transmission of null data as the guard interval increases decision error probability of a received OFDM symbol involving interference between subcarriers if the receiver incorrectly estimates the start of the OFDM symbol. Hence, the guard interval is inserted in the form of a cyclic prefix or a cyclic postfix. The cyclic prefix is a copy of predetermined last samples of a time-domain OFDM symbol, inserted into an effective OFDM symbol. The cyclic postfix is a copy of predetermined first samples of a time-domain OFDM symbol, inserted into an effective OFDM symbol.
The DAC 118 converts the output of the guard interval inserter 116 to an analog signal and outputs the converted signal to the RF processor 120. The RF processor 120, which includes a filter and front end units (not shown), RF-processes the analog signal to be transmitted over the air, and transmits the RF signal over the air through a transmit (Tx) antenna.
Now a description will be made of the receiver 150. The receiver 150 operates in the reverse order of the operation of the transmitter 100.
The receiver 150 comprises an RF processor 152, an analogue-to-digital (ADC) 154, a guard interval remover 156, an SPC 158, an FFT 160, a pilot symbol extractor 162, a channel estimator 164, an equalizer 166, a PSC 168, a symbol demapper 170, and a decoder 172.
A signal transmitted from the transmitter 100 experiences a multi-path channel and arrives at a receive (Rx) antenna as a signal having noise. The RF processor 152 downconverts the signal received from the Rx antenna to an intermediate frequency (IF) signal. The ADC 154 converts the analog IF signal to a digital signal.
The guard interval remover 156 removes a guard interval from the digital signal. The SPC 158 parallelizes the serial signal received from the guard interval remover 156 and the FFT 160 performs an N-point fast Fourier transformation on the parallel signals. The equalizer 166 channel-equalizes the FFT signal, and the PSC 168 serializes the equalized signal.
The pilot symbol extractor 162 detects pilot symbols from the FFT signal and the channel estimator 164 estimates a channel using the pilot symbols and provides the channel estimation result to the equalizer 166. The receiver 150 creates a CQI (Channel Quality Information) corresponding to the channel estimation result and transmits the CQI to the transmitter 100 through a CQI transmitter (not shown).
The symbol demapper 170 demodulates the serial signal received from the PSC 168 according to a predetermined demodulation method. The decoder 172 decodes the demapped symbols according to a predetermined decoding method and outputs the resulting final received user data 174. The demodulation and decoding methods are determined in correspondence with the modulation and coding methods used in the transmitter 100.
The OFDM system may use an OFDMA scheme to control multiple user accesses. The OFDMA scheme allows each user to use subsets of an OFDM subchannel through frequency hopping (FH) for spread spectrum. In OFDMA, a single user can transmit on a particular subchannel exclusively at any given time. In this environment, radio resources allocation is significant for optimization of system performance.
In the OFDM mode, an OFDM symbol is a basic unit for allocating resources. The number of bits of data that one OFDM symbol delivers is determined according to the modulation and coding scheme used as well as the number of data carriers per symbol. Meanwhile, the basic resources allocation unit is a subchannel in the OFDMA mode. Each OFDM symbol is transmitted on an integer number of subchannels according to the size of the FFT and the number of data bits per subchannel is equal to that of data carriers per subchannel.
It is well known that an OFDMA system is effectively implemented using channel status information. Conventionally, channel allocation is performed using signal to noise ratio (SNR) measured for a predetermined period based on radio channel status information. Or more simply, the best channel selected based on previous channel status information is first allocated.
To increase capacity using a given bandwidth in a multicell environment, a reuse partitioning method was proposed for a conventional FDMA system.
The reuse partitioning method is suited for the case where there are one modulation scheme, one service class, and one channel allocated per user. Therefore, it is not viable for the multicell environment of the OFDMA system in which adaptive modulation and coding (AMC) is used and users require various classes of data with different data rates. Moreover, a perfect dynamic channel allocation leads to complex implementation in view of cell coordination and channel estimation errors and increases performance degradation considerably.